1. Field of the Invention
The present invention relates to a method for manufacturing a pad used with an air bag device. The air bag device may also be installed on a steering wheel, in front of a passenger's seat, or on doors or seats of automobiles. The pad has a thin breakable tear seam portion which is severed upon inflation of the air bag.
2. Description of Related Art and Background
Conventionally, pads of this type are made of synthetic resin formed by injection molding. They have a thin breakable portion (sometimes referred to as a tear line) which breaks or splits open when the air bag inflates so that the air bag protrudes from the pad at the time of inflation. The breakable portion has a thinner notch portion for smooth breaking even if the pad is made of soft and elastic material.
The aforementioned breakable portion provided with the aforementioned notch portion is formed in the following manner, as disclosed in Japanese Patent Publication No. Hei. 2-248238. The pad is formed by injection molding, but initially without the notch portion. After that, the breakable portion is partially melted by heat to form the notch portion therein.
In this case, it is necessary to provide a mold cavity to form the pad with a narrow portion where the gap of the cavity is extremely narrow in order to form the notch portion simultaneously with injection molding of the pad. However, molding failure easily occurs when the narrow portion is provided in the cavity, because molding material cannot flow smoothly at the narrow portion. Therefore, injection molding which can provide the notch portion is very difficult.
However, when forming a notch portion at the breakable portion of the pad using a conventional heat melting method, a welding machine such as a high-frequency welder must be operated while controlling the thickness of the breakable portion. This is a laborious operation, so that the complexity of the process and cost of manufacturing the pad are increased.